Move Grafana plugins directory to default location

Summary:
Per principle of least astonishment, and lack of arguments /var/db
is suitable for HTML, CSS and JS resources.

Ref T651

Test Plan:

• tested on WindRiver by restarting Grafana
• nginx part checked through /public/plugins/.../img/... URL

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T651

Differential Revision: https://devcentral.nasqueron.org/D3379